Jewel__A_Celebration_of_Earth_s_Treasures

PHYSICAL PROPERTIES 017

Streak is the colour of the powdered mineral,

determined by rubbing a piece of the mineral across

a piece of unglazed porcelain, leaving a streak of

colour. This can be used for identification, particularly

with minerals that occur in different colours.

Fracture is another way of describing how a mineral breaks. In fracture,

however, breakage takes place across the mineral’s atomic planes, rather

than along them, as it does in cleavage, because there are no obvious

planes of fracture. Distinctive fracture may help with identification.

Conchoidal fracture is where the

breakage has a shell-like appearance.

Quartz and glass gemstones – such as

obsidian – show conchoidal fracture.

Even fracture has a broken surface

that is roughly textured, but flat.

Uneven fracture, as in the chalcopyrite

above, has a rough and irregular surface.

Hackly fracture shows an uneven

surface with sharp edges and jagged

points. It is characteristic of broken or

torn metals and a few other minerals.

Streak can be more

consistent in minerals

than colour – in fact,

a mineral that occurs

in different colour varieties

may have the same

colour streak. In this

example, three colour

varieties of the mineral

fluorite rough are shown.

In a streak test, the

purple, orange, and green

fluorite specimens would

all give a white streak.

KNOOP SCALE (kg/mm^2 )

Cleavage is the property of a mineral that

causes it to break along its atomic layers,

where the forces bonding its atoms are the

weakest. Some gems have cleavages in

several directions, some of which may be

very easy to trigger, meaning that the gem

can be easily broken if sharply knocked.

Because they follow the atomic planes,

cleavage surfaces are often smooth.

Perfect cleavage is the breakage of a mineral along

an atomic plane, where the bonds are weakest, and

where the breakage leaves a flat surface.

Aquamarine

Diamond

Obsidian Chalcopyrite Gold

Flat surface

Cuts glass

Jagged edges

Specific gravity (SG) is a measure of the

density of a substance relative to that of water,

and determines how dense a gemstone is.

It is measured as the ratio of the mass of the

substance and the mass of an equal volume

of water – so a mineral with an SG of 2 is twice

as heavy as water. Specific gravity can be

determined using specialized balances or

liquids that allow minerals below a given SG

to float and those above it to sink. However,

experts can often gauge the specific gravity

of a gem purely by its heft.

Emerald Tourmaline Dioptase

Emerald

(SG 2.78)

floats

Tourmaline

(SG 3.20)

half-floats

Dioptase

(SG 3.35)

sinks

High-density

liquid

Specific gravity

Cleavage Streak

Fracture

Green fluorite

leaves white streak

Orange fluorite

leaves white streak

Purple fluorite

leaves white streak

6000 7000

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